The present invention relates to an active matrix type liquid crystal display apparatus, in which an electric field is applied, mainly parallel to the substrates, and in which a broad visual field and a large numerical aperture are realized.
In a conventional liquid crystal display apparatus, the electrodes for driving the liquid crystal layer are formed on the surface of two substrates, and transparent electrodes are used, the electrodes facing each other. This is because what is used is a twisted nematic display method (TN method) in which the liquid crystal layer is driven by applying an electric field in a direction vertical to the surface of the substrate. On the other hand, a display method in which an electric field is applied in a direction almost parallel to the surface of the substrate using comb-type electrodes has been disclosed in the Japanese Patent publication No. 63-21907 (1988) and U.S. Pat. No. 4,345,249. In this case, the electrodes are not necessarily selected to be transparent, but non-transparent and metallic electrodes with a higher electric conductivity are used. However, in the above-mentioned prior art, for the display method in which the electric field is applied in a direction substantially parallel to the surface of the electrode, which is designated as a xe2x80x9chorizontal electric field methodxe2x80x9d, specifically used in the active matrix drive mode, the material property required to realize a high numerical aperture is not described in detail.
In the conventional active matrix type liquid crystal display apparatus, typically using the twisted nematic method, a transparent electrode is used. It is, therefore, possible to make the numerical aperture relatively wide, which is the area that the light per unit pixel passes through. However, in the horizontal electric field method, an opaque metal electrode is used. It is, therefore, impossible to realize a large numerical aperture. In this regard, there is an essential problem in that the opaque portion of the electrode can not be used as part of the area that light passes through. Further, the brightness of the display apparatus also depends upon the magnitude of the numerical aperture. Even if the intensity of the background light is increased in an effort to alleviate this problem, the problem arises that the power consumption is increased extremely.
Accordingly, in order to realize a large numerical aperture when employing the horizontal electric field method, it is necessary to enlarge the gap between the electrodes. However a new problem arises due to the enlargement of the gap. First, a disturbance of the orientation occurs due to static electricity, because the volume of the liquid crystal decreases further. Generally, the electrode in the horizontal electric field method is different in configuration from the one in the other type. Therefore, the volume of the liquid crystal is not so large. If the volume becomes smaller by enlarging the gap between the electrodes, the liquid crystal is susceptible to the effects of static electricity. As a result, the disturbance of the orientation due to static electricity increases. Secondly, if the gap between the electrodes becomes large, a problem arises in that the display method driven by the electric field requires a large drive voltage.
An object of the present invention is to provide an active matrix type liquid crystal display apparatus using the horizontal electric field method, in which a broad visual field and a large numerical aperture are realized.
In the present invention, the following features are used for solving the above mentioned problems and attaining the above objective.
According to a first aspect of the present invention; the present invention comprises a pair of substrates, at least one of which is transparent; a liquid crystal layer inserted between the substrates; an orientation film provided between the liquid crystal layer and at least one of the pair of substrates and on an inner side of said at least one substrate; a scanning signal electrode, an image signal electrode, a pixel electrode and an active device, each provided between the liquid crystal layer and the at least one substrate; and polarization means provided on the outer side of the substrates, for changing a optical characteristic according to an orientation state of the liquid crystal; each of said electrodes being constructed so as to apply an electric field, mainly parallel to said substrates, against said liquid crystal layer, and being connected to an external control means for controlling optionally the applied electric field according to the display pattern; wherein said electrode lies between at least two dielectric layers disposed above and below the electrode, and wherein the resistivity of said liquid crystal is higher than or equal to 1xc3x97109xcexa9.cm and lower than or equal to 1xc3x971013xcexa9.cm.
According to a second aspect of the present invention, the present invention comprises a pair of substrates, at least one of which is transparent; a liquid crystal layer inserted between the substrates; an orientation film provided between the liquid crystal and at least one of the pair of substrates and on an inner side of said at least one substrate; a scanning signal electrode, an image signal electrode, a pixel electrode and an active device, each provided between the liquid crystal layer and the at least one substrate; and polarization means provided on the outer side of the substrates, for changing a optical characteristic according to an orientation state of the liquid crystal; each of said electrodes being constructed so as to apply an electric field, mainly parallel to said substrates, against said liquid crystal layer, and being connected to an external control means for controlling optionally the applied electric field according to the display pattern; wherein a ratio 1/d of a gap 1 between said electrodes to a cell gap d is greater than or equal to 2.0, and the relation between an elasticity constant K2 and a dielectric anisotropy be satisfies the following equation (1):
K2/xcex94xcex94xcex5 less than 9.0xc3x9710xe2x88x928 [dyn],xe2x80x83xe2x80x83(1).
According to a third aspect of the present invention, in the arrangement of the second aspect, the gap between the substrates facing each other is made to be less than or equal to 6 mm, the gap between the electrodes is made to be more than or equal to 10 mm, and the drive voltage is made to be lower than or equal to 5V.
According to a fourth aspect of the present invention, in the arrangement of the first and second aspects, the liquid crystal composite material is made to include a liquid crystal chemical compound represented by a general chemical formula (I), in which a cyano group, trifluoromethyl group, trifluoromethoxyl group or nitro group is included as an end group: 
(In the formula (I), X1, X2 and X3 are a fluoro group, cyano group, trifluoromethyl group, trifluoromethoxyl group, nitro group or hydrogen atom; R is an alkyl group or alkoxyl group having a carbon number 1 to 10 which can be substituted; Ring A is a cyclohexane ring, benzene ring, dioxane ring, pyrimidine ring, or [2,2,2]-bicyclohexane ring; Z is a single bonding, ester bonding, ether bonding, or methylene; or ethylene; and n is an integer, 1 or 2.)
According to a fifth aspect of the present invention, in the arrangement of the first and second aspects, the liquid crystal composite material is made to include a liquid crystal chemical compound represented by a general chemical formula (II), in which a cyano group, trifluoromethyl group, trifluoromethoxyl group, or nitro group is included in a transverse axis of the molecule of the liquid crystal chemical compound: 
(In the formula (II), X1 and X2 are a fluoro group, cyano group, trifluoromethyl group, trifluoromethoxyl group, nitro group or hydrogen atom; R is an alkyl group or alkoxyl group having a carbon number 1 to 10 which can be substituted; Ring A is a cyclohexane ring, benzene ring, dioxane ring, pyrimidine ring, or [2,2,2]-bicyclohexane ring; Z is a single bonding, ester bonding, ether bonding, or methylene, or ethylene; and n is an integer 1 or 2.)
According to a sixth aspect of the present invention, in the arrangement of the first and second aspects, when the dielectric anisotropy of the liquid crystal is positive, the rubbing angle is set to an angle more than or equal to 1 degree and less than or equal to 20 degrees relative to the vertical direction of the electric field, and when the dielectric anisotropy of the liquid crystal is negative, the rubbing angle is set to an angle more than or equal to 1 degree and less than or equal to 10 degrees relative to the direction of the electric field.
According to a seventh aspect of the present invention, in the arrangement of the first and second aspects, the common electrode is composed of a part of the display pixels, and an alternating current is applied to the common electrode.
According to an eighth aspect of the present invention, in the arrangement of the first and second aspects, the transmission axis of said polarizer is set to an angle deviated more than or equal to 1 degree from an initial orientation direction of the liquid crystal to a rotation direction of the axis of the molecule of the liquid crystal due to the applied electric field.
FIG. 2 illustrates the angle xcfx86P defined by the polarization transmission axis, the angle fLC defined by the liquid crystal molecule longitudinal axis (optical axis) at the interface neighborhood, and the angle xcfx86R defined by the condensive axis in the phase shifter plate inserted between a couple of polarizers. The angle xcfx86P and xcfx86LC are expressed selectively in terms of xcfx86P1, xcfx86P2, xcfx86LC1, and xcfx86LC2 with regard to a couple of polarizers and a couple of liquid crystal interfaces.
FIGS. 1(a) and 1(b) show side cross-section views showing the operation of the liquid crystal in the liquid crystal panel, and FIGS. 1(c) and 1(d) show front views thereof in accordance with the present invention. In FIG. 1, active devices are not shown. In addition, though a plurality of pixels are formed with striped electrodes in accordance with the present invention, a partial view of a single pixel is shown. The side cross-section view when a voltage is not applied is shown in FIG. 1(a) and the front view is shown in FIG. 1(c). The linear electrodes 3 and 4 are formed inside a couple of transparent substrates 7, and a couple of orientation films 5 are coated on the substrates 7 so that they face each other. Liquid crystal composite material is inserted between the films. The liquid crystal molecule 6 shaped in line is oriented so that the angle fLC of the longitudinal axis of the molecule between the longitudinal direction of the Y electrode, shaped in the form of a stripe, may be maintained to be at an adequate angle, such as 45xc2x0 less than |xcfx86LC| less than 90xc2x0.
For the explanation below, the orientation directions of the liquid crystal molecule on each of the upper and lower interfaces are assumed to be parallel to each other, that is, fLC1=fLC2.
The dielectric anisotropy of the liquid crystal composite material is assumed to be positive. Next, when the electric field is applied, as shown in FIGS. 1(b) and 1(c), the axis of the liquid crystal molecule becomes oriented in the direction of the electric field. By placing the polarizer 8 on the polarization transmission axis, the optical transmission index can be modulated by applying and changing the electric field. Thus, a display operation for defining contrast is accomplished without transparent electrodes. Though the dielectric anisotropy of the liquid crystal composite material is assumed to be positive, it may be selected to be negative. In the case of a negative dielectric anisotropy, for the initial orientation of the liquid crystal molecule, the angle fLC is maintained at an adequate angle to the vertical direction with respect to the longitudinal axis of the striped electrode, such as 0xc2x0 less than |xcfx86LC|45xc2x0.
By providing the electrode between at least two dielectric layers disposed above and below the electrode, and by making the resistivity of the liquid crystal lower than or equal to 1xc3x971013xcexa9.cm, as described in the first aspect, it is possible to increase the numerical aperture. The reason for this will be explained hereinafter. In generating the horizontal electric field, an opaque metal electrode is used. It is, therefore, impossible to realize a numerical aperture larger than the one of the prior art. The basic method of solving this problem is to enlarge the gap between the electrodes. However, a new problem may arise due to such enlargement of the gap. The disturbance of the orientation occurs due to static electricity, because the volume of the liquid crystal decreases further. To provide an auxiliary capacitance for each pixel brings about a deterioration of the numerical aperture. However, if the resistance of the liquid crystal is low, the disturbance of the orientation due to static electricity is small. Therefore, this provides a very effective solution. By this means, the domain around a spacer bead is also improved. In the conventional active matrix type liquid crystal display apparatus, it is required to use a liquid crystal with a high resistivity of at least 1xc3x971013 xcexa9.cm, preferably, 1xc3x971014xcexa9.cm, in order to apply a sufficient voltage even during a non-selected period of time. In the horizontal electric field method the dielectric, except for the liquid crystal, such as a glass or other insulation film, acts to hold a capacitance to obtain the voltage hold ratio necessary for proper operation. By experiments, we found that even a liquid crystal with a resistivity of 1xc3x971010xcexa9.cm has a high voltage hold ratio (frame frequency: 60 Hz) of more than 90%. However, compared to a conventional method, such as the TN method, the total capacitance, including the capacitance of the liquid crystal and the hold capacitance, is smaller in magnitude in the horizontal electric field method and is susceptible to the effect of static electricity. Not only the problem of static electricity, but also an increase in the drive voltage is brought about by an enlargement of the gap between the electrodes.
As for the means for reducing the drive voltage, it is found to be effective that, as described in connection with the second and third aspects, the relation between the dielectric anisotropy (xcex94∈) of the liquid crystal composite material and the elasticity constant (K2) of the twist should satisfy the equation, K2/xcex94∈ less than 9.0xc3x9710xe2x88x928 [dyn]. As the thickness of the electrode is less than that of the liquid crystal composite material layer in the ordinary horizontal electric field method, it is impossible to apply an electric field which is completely parallel to the interface between the liquid crystal layer and the orientation layer onto the liquid crystal composite material layer. This insufficient horizontal electric field tends to cause a reduction in the efficiency of switching operation of the liquid crystal on the interface. By making the dielectric constant ∈LC Of the liquid crystal layer larger than the dielectric constant ∈AF of the orientation layer, preferably by making ∈LC twice as large as ∈AF, a horizontal electric field which is more parallel to the interface between the liquid crystal and the orientation layer can be provided. With more intensive study in which the elasticity constant K2 of the twist is modified to be smaller, or the dielectric anisotropy of the liquid crystal is modified to be larger, when the ratio between the elasticity constant and the dielectric anisotropy is set to be 9.0xc3x9710xe2x88x928 [dyn], and more, preferably when this ratio is set to be 7.0xc3x9710xe2x88x928 [dyn], a voltage drive of 5V or less can be attained. A drive voltage of 5V means that it is possible to produce a display by using the 5V voltage-tightness of a signal voltage driver.
As described in connection with the fourth aspect, by using a liquid crystal chemical compound represented by a general chemical formula (I), in which a cyano group, trifluoromethyl group, trifluoromethoxyl group or nitro group is included as an end group, we obtained an effective liquid crystal with a large numerical aperture. Namely, by a decrease in the resistivity, the static electricity can be prevented, and at the same time the drive voltage can be decreased. The decreasing of the resistivity is further effective for a small viscosity necessary for high-speed response. The liquid crystal which can not increase its resistivity, such as cyano groups, can be used for the display apparatus, because liquid crystals with lower resistivity show a high voltage hold ratio in the horizontal electric field method. Therefore, the margin of the kind of the liquid crystal to be used remarkably increases. 
(In the formula (I), X1, X2 and X3 are a fluoro group, cyano group, nitro group or hydrogen atom; R is an alkyl group or alkoxyl group having the carbon number 1 to 10 which can be substituted; Ring A is a cyclohexane ring, benzene ring, dioxane ring, pyrimidine ring, or [2,2,2]-bicyclooctane ring; Z is a single bonding, ester bonding, ether bonding, or methylene, or ethylene; and n is an integer 1 or 2.)
More concretely, there is suggested 1,2-dicyano-4-[trans-4-(trans-4-propylcyclohexyl) cyclohexyl]benzene; Trans-4-propyl-(3,4-dicyanobiphenyl-4xe2x80x2-yl)cyclohexane; 2-(trans-4-propylcyclohexyl)-1-[trans-4-(3,4-dicyanophenyl)cyclohexyl]ethane; 3,4-dicyanophenyl-trans-4-bentylcyclohexylcarboxylate; 4-cyano-3-fluorophenyl-trans-4-propylcyclohexylcarboxylate; trans-4-heptyl-(3,5-difluoro-4-nitrophenyl)cyclohexane; 2,6-difluoro-1-cyano-4-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]benzene; trans-4propyl-(3,4,5-trifluorobiphenyl-4xe2x80x2-yl)cyclohexane; 2-(trans-4-propylcyclohexyl)-1-[trans-4-(3,5-difluoro-4-nitrophenyl)cyclohexyl]ethane; 3,5-difluoro-4-nitrophenyl-trans-4-bentylecyclohexylcarboxylate; trans-4-heptyl-(3-fluoro-4-cyanophenyl)cyclohexane; 2-fluoro-1-nitro-4-[trans-4(trans-4-propylcyclohexyl)cyclohexyl]benzene; trans-4-propyle-(3-fluoro-4-cyanobiphenyl-4xe2x80x2-yl)cyclohexane; 2-(trans-4-propylcyclohexyl)-1-[trans-4-(3-fluoro-4-nitrophenyl)cyclohexyl]ethane; 3-fluoro-4-cyanophenyl-trans-4-pentylecyclohexylcarboxylate; trans-4-heptyl-(4-cyanophenyl)cyclohexane; 4-cyanophenyl-5-bentyl-1,3-pyrimidine; 4-cyano-3-fluorophenyl-5-propyl-1,3-pyrimidine; 4-cyanophentl-4-pentyl-1,3-dioxane; 4-cyanophenyl-4-pentyl-[2,2,2]-bicyclooctane. The present invention does not exclude compounds other than the above. The liquid crystal compound having a fluoro group at the ortho position in its cyano end group, represented by 4-cyano-3-fluorophenyl-trans-4-propylcyclohexylcarboxylate, is known to be a material which does not tend to form dimer to cancel the dipole momentum. As such liquid crystal compound has a larger dielectric constant and a lower viscosity, it is effective to apply this kind of compound to the high-speed driving operation in the horizontal electric field method.
When using a liquid crystal having a negative dielectric anisotropy, by including a liquid crystal chemical compound represented by a general chemical formula (II), in which a cyano group, trifluoromethyl group, trifluoromethoxyl group or nitro group is included as an end group, as described in connection with the fifth aspect, we obtained an effective liquid crystal with a large numerical aperture. Namely, by a decrease in the resistivity, the static electricity can be prevented, and at the same time the drive voltage can be decreased. The decreasing of the resistivity is further effective for a small viscosity necessary for high-speed response. The liquid crystal which can not increase its resistivity, such as the cyano groups, can be used for the display apparatus, because liquid crystals with a lower resistivity show a high voltage hold ratio in the horizontal electric field method. Therefore, the margin of the kind of liquid crystal to be used remarkably increases. 
(In the formula (II), X1 and X2 are a fluoro group, cyano group, nitro group or hydrogen atom; R is an alkyl group or alkoxyl group having the carbon number 1 to 10 which can be substituted; Ring A is a cyclohexane ring, benzene ring, dioxane ring, pyrimidine ring, or [2,2,2]-bicyclooctane ring; Z is a single bonding, ester bonding, ether bonding, or methylene, or ethylene; and n is an integer 1 or 2.)
More particularly, there is inputted trans-4-heptyl-(2-cyano-3-fluorophenyl)cyclohexane; 2-cyano-3-fluoro-4-[trans-4(trans-4-propylcyclohexyl)cyclohexyl]benzene; Trans-4-propyl-(2-cyno-3-fluorobiphenyl-4xe2x80x2-yl)cyclohexane; 2-(trans-4-propylcyclohexyl)-1-[trans-4-(2-cyano-3-fluorophenyl)cyclohexyl]ethane; 2-cyano-3-fluorophenyl-trans-4-pentylecyclohexylcarboxylate; trans-4-heptyl-(2-fluoro-3-nitrophenyl)cyclohexane; 2-fluoro-3-cyano-4-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]benzene; trans-4-propyle-(2-fluoro-3-nitrobiphenyl-4xe2x80x2-yl)cyclohexane; 2-(trans-4-propylcyclohexyl)-1-[trans-4-(2-fluoro-3-nitrophenyl)cyclohexyl]ethane; 2-fluoro-3-cyanophenyl-trans-4-bentylecyclohexylcarboxylate; 2,3-dicyanophenyl-5-pentyl-1,3-pyrimidine; 2-cyano-3-fluorophenyl-5-propyl-1,3-pyrimidine; 2,3-dicyanophenyl-4-pentyl-1,3-dioxane; 2-cyano-3-fluorophenyl-4-pentyl[2,2,2]-bicyclooctane. The present invention does not exclude compounds other than the above.
As described in connection with the sixth aspect, when the dielectric anisotropy of the liquid crystal is positive, the rubbing angle is set to an angle more than or equal to 1 degree and less than or equal to 10 degrees relative to the vertical direction of the electric field, and when the dielectric anisotropy of the liquid crystal is negative, the rubbing angle is set to an angle more than or equal to 1 degree and less than or equal to 10 degrees relative to the direction of the electric field. This is another important way of reducing the drive voltage. As the angle between the axis of the molecule of the liquid crystal and the direction of the electric field becomes close to 90 degree, the voltage at the maximum transmission factor shifts to the low voltage side. Further, a non-select voltage can be set to the high voltage side, because the threshold voltage of the response to the electric field shifts to the high voltage side. Accordingly, it is possible to reduce the width of the drive voltage. FIG. 9 shows a typical example thereof, where it is desired that the rubbing angle is as small as possible, so long as any domains do not occur.
Further, it is possible to reduce further the drive voltage, by applying an alternating current to the common electrode, as described in connection with the seventh aspect, after the above sixth aspect was carried out.
Furthermore, we found that the eighth aspect is also effective to reduce the width of the drive voltage, as shown in FIG. 10, in which the transmission axis of a polarizing plate is deviated more than 1 degree from the initial orientation direction.